Submersible turbines running on compressed gas, typically air, are known. See for example U.S. Pat. No. 272,656 to Cook, U.S. Pat. No. 271,040 to Cook, and U.S. Pat. No. 211,143 to Fogarty. As turbines of this type run on low pressure gas, they are suitable for extracting energy from sources such as low pressure gas wells that are not otherwise useful. Moreover, such turbines are especially suited for use in environments where electric and combustion engines would be unsafe, as for example in the presence of explosive gases. The compressed air used to run the turbine may also be used to provide necessary ventilation. However, prior art devices have suffered from certain problems.
The submerged wheel is often provided with a housing that affords substantial clearance between the wheel and the housing. The relatively large housing relative to the wheel size results in an extremely heavy machine, possibly unsuitable for installation in existing structures, and further represents a substantial cost for material and fabrication.
This large clearance also results in the escape of air in the form of large bubbles that are released as the turbine wheel rotates. Once the air is clear of the turbine wheel, the upward motion of the air pocket is no longer available for driving the wheel. Moreover, as large amounts of water flow in to replace the air, large scale turbulence is created and extends to the surrounding water, thereby dissipating energy and reducing the efficiency. Local turbulence is also created, and produces undesirable vibrations of the turbine. As the upwardly moving air leaves the vicinity of the rotating turbine wheel, the inflowing water tends to replace the air in a relatively nondirectional manner. This inflow often acts in a direction opposite that in which the wheel is rotating, thereby producing vibration and a reduction of the energy output.
In order to increase the rotational range over which air is trapped in the chambers, backswept vanes are often used. However, this solution to one problem is itself the cause of other problems arising from the relatively closed configuration of the chambers. This manifests itself at the bottom of the wheel where gas that is injected must bubble up through the water in order to displace water downwardly out of the chamber, and at the top of the wheel where the gas must escape upwardly as water gravitationally fills the chamber. With the backswept vanes, additional resistance is encountered by the displaced water at the bottom and by the inflowing water at the top. Accordingly, it is not uncommon for turbines to operate with a considerable amount of air within the downwardly moving chambers on the wheel. This can occur either as a result of injected air's being unable to properly displace water from the lower chambers and thereby bubbling up the wrong side, or trapped air in the upper chambers being unable to escape quickly. In either case, the useful power that may be extracted from the turbine is reduced.
A further difficulty often encountered with prior art devices relate to loading on the axle bearings due to the weight of the wheel. Excessive loading leads to frictional losses and possible ultimate failure of the bearings themselves.